Retired Marine General James Cartwright is being investigated in connection with a leak of classified information about a covert U.S. cyber attack, legal sources say. But Cartwright's lawyer, Gregory Craig, said in a statement Cartwright is a hero who loves his country and wouldn't betray it. NBC's Michael Isikoff reports.

By Michael Isikoff, National Investigative Correspondent, NBC News

The four-star general who is the target of a leak investigation suddenly resigned from his Pentagon advisory job in January for “health reasons” – just as FBI investigators were focusing on him, according to legal sources.

As NBC News revealed Thursday, retired Marine Gen. James “Hoss” Cartwright has received a target letter informing him that he’s under investigation for allegedly leaking information about a massive U.S. cyberattack on Iran’s nuclear facilities using the computer worm Stuxnet, according to legal sources.

Cartwright was vice chairman of the Joint Chiefs of Staff, the nation’s second-highest-ranking military official, when he retired from the military in August 2011. He continued to serve after retirement as a member of the Defense Policy Board, an elite Pentagon advisory panel with top-secret national security clearance.

In January, he resigned from the board “out of the blue,” a board member told NBC News, due to health reasons and the “press of business.”

A spokesman for Cartwright, however, told NBC News that his departure from the advisory panel was not connected to the Justice Department leak probe. “The resignation from the DPB was unrelated to the investigation,” said the spokesman.

After a July 2012 New York Times article that disclosed details of the Stuxnet operation sparked an outcry from congressional leaders, Attorney General Eric Holder launched an investigation into who had leaked the information to the paper. Republicans had charged that senior administration officials leaked the details to burnish President Barack Obama’s national security credentials in the run-up to last November’s election.

Legal sources said that while the investigation initially focused on any White House connections, by late 2012 FBI agents were zeroing in on Cartwright. He had served as one of the president’s “inner circle” of national security advisers, and was described in the Times article as the architect of the “Olympic Games” operation, which had introduced the Stuxnet virus into Iran’s Natanz nuclear facility.

Cartwright has not commented on the probe, and Holder on Friday declined an NBC News request for comment.

But Cartwright’s attorney, former Obama White House counsel Greg Craig, issued a statement defending his client’s patriotism. “General Jim Cartwright is an American hero who served his country with distinction for four decades,” said Craig. “Any suggestion that he could have betrayed the country he loves is preposterous.”

Former colleagues said they were stunned to learn that Cartwright had been targeted.

“He’s dedicated his life to protecting the American people in so many different ways, and I find it shocking that he would be under investigation,” said Ellen Tauscher, a former Californian congresswoman and undersecretary of state who now serves on the Defense Policy Board.

Chip Somodevilla / Getty Images North America

Retired Marine Gen. James Cartwright, former vice chairman of the Joint Chiefs of Staff, attends a congressional hearing on Oct. 1, 2009.

Cartwright, 63, a former Marine Corps fighter pilot with the call sign “Hoss,” served in the Marines for 40 years. He rose to the rank of four-star general, and was the chair of the U.S. Strategic Command from 2004 to 2007 before becoming vice chairman of the Joint Chiefs of Staff.

He was known as a politically savvy pioneer in new forms of warfare, working closely with White House aides in selecting targets for drone strikes in Pakistan, but his real mark was in developing the nation’s cyberwarfare capabilities, both offensive and defensive.

“He was a crucial voice in communicating both the opportunities and dangers of cyber warfare,” said Peter Singer, a national security expert at the Brookings Institution think tank.